1. Field of the Invention
The present invention relates to a hot stand-by switching unit for a receiving apparatus composed of a regular and a protection receiver.
2. Description of the Related Art
In a switching unit of this kind, a hybrid type branch method has hitherto been adopted. For example, as shown in FIG. 1, a hybrid unit 12 is provided on the input side of a regular receiver 3 and a protection receiver 4, and a switching circuit 6 is provided on the output side thereof. Input signals are branched by hybrid unit 12 and are supplied to regular receiver 3 and protection receiver 4. Switching circuit 6 on the output side is switched by control circuit 13 which operates according to monitor signals of regular receiver 3 and protection receiver 4, and secures the reception of signals by selecting the protection receiver when the regular receiver breaks down.
As shown in FIG. 2, there is another conventional technique, in which a branch circuit 11 composed of PIN diode 8, 9 is disposed as a branch circuit and by controlling a bias current, it varies a degree of coupling with regular and protection receivers 3, 4 (Japanese Patent Laid-open. 268024/90). Specifically, when regular receiver 3 is normal and switching circuit 6 selects the regular receiver, the high-frequency resistance of PIN diode 8 is maintained at a high level without the bias current flowing through PIN diode 8 in order to keep the signal loss to be given to regular receiver 3 to a minimum. At the same time, a fixed amount of the bias current is passed through PIN diode 9 to maintain a leak of a certain amount, for example, about 10 dB to secure self monitoring of its own operation in protection receiver 4.
When regular receiver 3 breaks down, control circuit 10 switches switching circuit 6 according to the monitor signal of regular receiver 3 to select protection receiver 4. At this time, by flowing the bias current of a reverse condition against the above in PIN diodes 8, 9 in branch circuit 11, it becomes possible to suppress the insertion loss of protection receiver 4 to a minimum.
With the above conventional hot stand-by switching unit shown in FIG. 1, since hybrid unit 12 is used to branch received signals and the hybrid unit produces at least 3 dB branch loss, it causes a large degree of deterioration with reference to the performance of the system. Therefore, in the case of a receiving circuit having a protection receiver, compared with a receiving circuit having no protection receiver, signal input to the receiver is reduced by the loss due to the hybrid unit, here, by 3 dB or more.
In the conventional example shown in FIG. 2, PIN diodes 8, 9 are used in the branch circuit to vary the degree of coupling with regular receiver 3 and protection receiver 4 in order to suppress the signal loss. However, a PIN diode is an active element and hence has a generally high failure rate. In particular, since the branch circuit constitutes a common part at a preceding stage of the regular receiver and the protection receiver, if a failure occurs here, the failure can not be prevented by either the regular receiver or the protection receiver, resulting in serious signal discontinuation trouble. Therefore, using parts having a high failure rate such as a PIN diode in the branch circuit causes the degradation of reliability of the entire system.